SISTER-IN-LAW Meaning of SISTER-IN-LAW in English [sis.ter-in-law] n, pl sis.ters-in-law (15c) 1: the sister of one's spouse 2. a: the wife of one's brother b: the wife of one's spouse's brother Merriam-Webster English vocab. Английский словарь Merriam Webster. 2012